Hail

There are three different phenomena which affect the UK that can loosely be called hail.

1. Snow pellets that are white but are easily crushable between the fingers. They are occasionally called 'soft hail'.
2. Ice pellets that are quite moderate in size and are composed of clear ice, sometimes conical in shape.
3. Hailstones that are whitish in appearance and vary greatly in size. If a hailstone is cut open, a layered structure like an onion is sometimes apparent.

Large hailstones fall from deep cumulonimbus clouds. The cloud base may be 3,000 feet (900 m) above the ground with tops as high as 60,000 feet (18,000 m). Much of the cloud will be composed of supercooled water droplets. As the hailstone falls it will collect water droplets which instantly freeze and form a layer of ice. It may then be caught in a vigorous updraught and, as it is carried back higher into the cloud, it collects more water droplets or ice particles to form another layer of ice. Thus layers build up on the hailstone (made of alternate layers of clear and opaque ice) and the cycle may be repeated until the stone is so big that updraughts can't support it and it falls to earth.

Hail showers are relatively common over the UK in showery airstreams in spring, but really large hailstones tend to occur in thunderstorms that have originated from hot, continental air and are very much a feature of summer months.

The largest hailstone ever recorded in the UK weighed 141 grams (5 oz) and occurred at Horsham, West Sussex on 5 September 1958. Certainly anything approaching golf-ball size is remarkable, but hailstones can grow large enough to dent cars, shatter greenhouses, injure, and perhaps even kill people.

North America, central Europe, India and China all experience large hail. So too do land areas in the southern hemisphere. The heaviest hailstone (as quoted in the Guinness Book of Records) occurred in a hailstorm in the Gopalanj district of Bangladesh on 14 April 1986. The hailstones weighed up to 1 kg (2 lb 3 oz) and were reported to have killed 92 people.